1. Technical Field
The invention relates to the manufacture of a semiconductor structure having an optimum dielectric isolation and, more particularly, to a method of forming recessed oxide isolation structure with reduced steepness of the bird's neck in a silicon substrate.
2. Description of the Prior Art
In the semiconductor integrated circuit production, it is usually necessary to isolate various active and/or passive devices from one another in the integrated circuit structure. These devices have been isolated by back biasing PN junctions, partial dielectric isolation and complete dielectric isolation. The dielectric materials used have been silicon dioxide and the like. The preferred isolation for these active devices and circuits is some form of dielectric isolation which may include a combination with PN junction isolation. The dielectric isolation has the substantial advantage over the PN junction isolation because it allows the butting of the circuit elements against the isolation and thereby results in greater density of packing of the active and/or passive device on the integrated circuit chip.
One form of dielectric isolation is the Recessed Oxide Isolation (ROI). This approach which dates back to the early 1970's, involves chemically etching grooves or recesses in silicon where the isolation regions are to be formed. During the groove formation, the remainder of the silicon surface is protected by a mask called the ROI mask which is substantially unaffected by the etchant used to form the grooves in the silicon. Following the formation of the grooves the silicon body is subjected to thermal oxidation whereby the silicon in the groove area is converted to silicon dioxide filling up the groove as well as oxidizing further into the silicon to form the recessed oxide isolation region. Thus, in ROI formation, the ROI mask is first used as an etch barrier while recesses are etched through the mask openings in the silicon and is then used as an oxidation mask during thermal oxidation of the recesses. A composite silicon dioxide-silicon nitride mask has been widely used in this dual role. The ROI process is more fully described by U.S. Pat. Nos. 3,970,486 to E. Kooi, 3,534,234 to Peltzer and U.S. patent application Ser. No. 150,609 by I. Magdo et al. filed June 7, 1971.
Recently, the formation of grooves in silicon and filling them with a dielectric has been significantly improved by using other materials for the ROI mask than oxide-nitride composite, by substituting wet etching with dry reactive ion etching (RIE) technique to etch the grooves, and by filling the grooves with a dielectric material other than oxide to form the device isolation. In this context, reference is made to the article entitled "Elimination of Reactive Ion Etching trench induced defects", by G. Das et al, IBM Technical Disclosure Bulletin, Vol. 23, No. 12, page 5344, May 1981, and U.S. Pat. No. 4,104,086 to J. A. Bondur et al. and assigned to the present assignee.
One of the major problems associated with the above ROI process is what is known as "bird's beak". The "bird's beak" is a silicon dioxide formation at the top periphery of the groove and is caused by the lateral oxidation underneath the silicon nitride layer. Since the oxidation of a specific thickness of silicon requires an almost equivalent amount of free space to expand into, and since the nitride limits the unrestricted expansion, the result is an up-pushing of the silicon nitride at the edge of the groove. Associated with the bird's beak is the bird's head, an undesirable bump in the order of 400-500 nm in height formed in the edge of the thick oxide layer adjoining the oxide-nitride mask. For a more complete understanding of the theory behind the bird's head and bird's beak phenomena, reference is made to U.S. Pat. No. 3,900,350 to J. A. Appels et al. Associated sometimes with the bird's head is an excessively steep bird's neck stemming from excessive etching of the planar surface portions of the ROI during removal of the ROI mask.
The existence of the bird's head or bird's beak incurs many disadvantages. The device active area is reduced and the isolation area is increased by the bird' beak. This is disadvantageous since chip real estate, particularly in very large scale integrated (VLSI) circuits, is at a premium. Because of the bird's beak, it is difficult to achieve well-defined lateral isolation boundaries. Because of the bird's head, the top surface of the resulting ROI structure will not be substantially planar with the top surface of the silicon. As a result, electrode interconnections formed on the ROI regions are prone to be fractured and disconnected. Another drawback of nonplanarity is poor coverage of metal lands over the corners and edges of the surface topography leading to unreliability of the integrated circuit. Yet another disadvantage is that the nonplanar surface would not be conducive for multi-level conductive personalization demanded by high performance VLSI circuits. A further problem, which is a direct result of an excessively steep bird's neck and which is discussed more fully in the article entitled "Preventing Formation of Polysilicon Rails" by C. G. Jambotkar, IBM Technical Disclosure Bulletin, Vol. 25, No. 12, pages 6606-6609, May 1983, is the formation of polysilicon rails after forming a polysilicon layer on the nonplanar surface and etching the polysilicon by RIE into polysilicon base and/or emitter contacts of a bipolar transistor. The polysilicon rails tend to electrically short-out adjacent devices destroying the benefit of the ROI.
The foregoing are representative of the numerous disadvantages associated with the bird's beak and bird's head. Thus, it is readily apparent that it is desirable to reduce or eliminate the bird's beak and bird's head associated with ROI. Reduction in bird's head may, in general, result in some reduction in the steepness of the bird's neck.
Attempts have been made in the prior art to reduce the bird's beak and bird's head problems. For example, the J. A. Appels et al. U.S. Patent cited hereinabove describes a method to overcome the bird's beak problem. This patent suggests the substitution of a layer of polycrystalline silicon for the silicon dioxide layer between the silicon nitride masking layer and the monocrystalline silicon body. The patent states that the oxidation of monocrystalline silicon is not noticeably different from oxidation of polysilicon. However, the bird's beak is greatly reduced by introducing the polysilicon as a buffered layer between the oxide and the nitride. This polysilicon layer was also found to dampen the mechanical stress caused by the silicon nitride when directly applied to the silicon.
U.S. Pat. No. 4,272,308 to R. C. Varshney discloses deposition on the oxide-nitride dual mask including the sidewalls thereof, prior to thermal oxidation, a second nitride layer. The second nitride serves to seal off lateral oxidation underneath the first nitride during the thermal oxidation of the exposed silicon thereby reducing the bird's beak.
U.S. Pat. No. 4,170,500 to P. A. Crossley discloses a process of forming dielectric isolation by first forming on the silicon substrate a dual oxide-nitride layer. Then, polysilicon is formed on the surface of the nitride layer in region corresponding to where device isolation is required. The polysilicon is next fully oxidized followed by etching off the exposed oxide-nitride dual mask leaving a stack of oxide-nitride-oxide serving as dielectric isolation free of bird's beak/head.
European Patent Application No. 82106651.1 (publication No. 0 071 203) by B. M. Kemlage and assigned to the present assignee discloses a method of reducing the bird's head. In this method the oxide pad in the oxide-nitride dual layer is replaced with a silicon oxynitride (SiO.sub.x N.sub.y) layer. The oxynitride acts as a barrier to rapid oxidation during the thermal oxidation step, thereby reducing the bird's head.
The article entitled "Fabrication of Oxide Isolation Using an Oxynitride/Polysilicon Mask" by B. M. Kemlage et al., IBM Technical Disclosure Bulletin, Vol. 24, No. 9, February 1982, page 4756, proposes an oxynitride/polysilicon stack mask. The oxynitride while serving as an effective oxidation mask imposes lesser stress on the silicon than nitride during thermal oxidation. The polysilicon serves as a buffer inhibiting oxidation under the oxynitride thereby eliminating the bird's beak and excessive bird's head.
The article entitled "Method of Manufacturing Dielectrically Isolated Regions of Silicon Utilizing High Pressure Steam" by D. W. Ormond, IBM Technical Disclosure Bulletin Vol. 23, No. 8, January 1981, pp 3694-3697 discloses use of a polysilicon layer sandwiched between the pad oxide and nitride layers of the ROI mask in the context of high pressure oxidation of the silicon to form ROI. The polysilicon is believed to reduce the large stresses caused by the nitride during high pressure oxidation and also reduce the bird's beak.
Despite the improvement in suppressing the bird's beak and bird's head exemplified by the above prior art, there is need for further improvement since these prior art attempts have been basically deficient. The deficiencies arise because the prior art attempts either to not completely solve the problem or else introduce additional problems such as defects at the silicon surface under the ROI mask or give rise to difficulties in implementing the processes in a high volume manufacturing environment, the latter principally due to lack of controllability of the process.
Reference is now made to U.S. Pat. No. 4,508,757 to K. A. Fabricius, et al. and assigned to the present assignee which discloses a method for reducing the bird's beak. In accordance with the Fabricius et al. process, which in relevant part, can be understood with reference to FIG. 1, ROI with reduced beak formation is obtained by starting with a P-type monocrystalline silicon body 10 having an N-type epitaxial layer 11 on the top surface thereof. A layered structure of silicon dioxide 12, polysilicon 13 and silicon nitride 14 are formed, in that order, on the epitaxial layer 11. The thickness of the oxide layer 12 is about 2-10 nm, polysilicon 13 is about 5-500 nm and nitride 14 is about 10-500 nm. The layers 12-13-14 are then patterned by conventional lithography and RIE to form openings in the structure at the areas where it is desired to form an oxide isolation pattern within the body 10. Then, grooves are formed in the epi layer 11, preferably, by continuing the RIE process utilized to define the openings in the layers 12-13-14. The resulting structure is now subjected to a high temperature oxidizing treatment forming the ROI regions 15. The nitride 14 is then removed by a hot phosphoric acid dip etching solution. The polysilicon 13 is then either etched off in full or partially etched followed by conversion of the remaining polysilicon into an oxide layer (not shown) atop the oxide 12.
The Fabricius et al. process, although substantially reduces the beak length 16 and bird's head or crest height 17, requires arduous process control lest a rather pointed bird's head or steep bird's neck 18 is obtained which would lead, among other things, to the polysilicon rail problem previously discussed. The reason for this undesirably steep neck 18 is the formation of a thin silicon oxynitride layer (not shown) on the top of the polysilicon 13 during the initial phase of the nitride 14 deposition. The oxynitride does not lend itself to easy removal, following the ROI 15 formation, causing the thinning and consequent steepening of the bird's neck. Also, the oxynitride inhibits oxidation of the polysilicon 13 during the final process step to convert the polysilicon 13 into an oxide layer.
It is an object of the invention to provide a method of forming ROI regions having reduced bird's beak and bird's head.
It is an object of the invention to provide a controllable method of forming ROI regions having bird's neck of a reduced steepness.
The above objects and other related objects and advantages are achieved through the use of a novel ROI mask and novel combination of etch steps.